Literally, "ungulate" refers to any animal with hooves - a hoof being an
enlarged toenail (see below). However, in practice, the use of the name
"ungulate" has been inconsistent. While it was originally used to refer to
the orders Artiodactyla (even-toed) and Perissodactyla (odd-toed) - the "true"
ungulates - over time the term expanded to seven different extant Mammalian
orders . . . some of which have no hooves whatsoever! This broadening of
the definition was based on presumed family relationships - relationships
that recent advances have shown to be artificial. As a result,
ungulate is now understood to have no taxonomic significance, and
its definition has returned to its original descriptive roots: a mammal
with hooves.

COUNT THE HOOVES: some ungulates
and their feet.

What is a HOOF?

A hoof is really just a modified toenail. Hooves, claws, and nails are all
composed of two structures: the unguis (a scale-like plate; our finger-
and toe-nails) and the subunguis (a softer layer, found as a very
fine layer on the underside of our fingernails) which connect the unguis
to the pad of the digit. Unlike claws and nails, hooves are the principal
point of contact between the legs and the ground - as a result, ungulates
are said to have unguligrade limbs.

In ungulates, the tough unguis encircles the tip of the digit as a cylinder,
enclosing the subunguis within in. Since the unguis is harder than the subunguis,
it does not wear down as quickly, resulting in a firm (sometimes sharp) leading
edge to the hoof. The pads of the digits lie behind the hooves - these pads
touch the ground in perissodactyls and some artiodactyls, while only the
hooves bear weight in other ungulate artiodactyls (notably the Suiformes
and Ruminantia).

In many artiodactyls, only the central two digits touch the ground. The
lateral digits are often reduced (if they are present at all) and only touch
the ground in soft terrain. Because of this, these lateral digits are called
"false hooves" or "dewclaws".

Why hooves?

The development of hooves illustrates a major innovation in the evolution
of a cursorial (running) lifestyle, pushed on early ungulates by fast-running
predators. Hooves have such an extensive structure because they support all
of the body weight of ungulates. Why is this important? Since only the hooves
touch the ground, the rest of the parts of the foot have essentially become
parts of the leg, substantially increasing the length of stride. Modern
ungulates have taken this to the extreme: the metapodials (the bones between
the wrist/ankle and the digits) are often as long as the other parts of the
legs. Raising the heel and digits off of the ground also increases the number
of joints which move the legs forward - this increases the rate of
stride. Coupled together, these two factors have given modern ungulates
the speed needed to survive, an evolutionary imperative which would not have
been possible without the development of hooves.

UNGULATE: A mammal with HOOFS or
HOOVES?

The 1964 Edition of Webster's Dictionary gives the plural of hoof as
"HOOFS, rarely HOOVES". Over the past few decades,
usage patterns of the plural forms of hoof have changed: the online
Merriam-Webster's Dictionary (2006) gives the plural as
"HOOVES, also HOOFS." Officially, either is
correct!

What
is an UNGULATE?The Historical Perspective

Anatomy and paleontology have historically been the main tools used to determine
evolutionary affinities. The two orders of true ungulates were grouped together
early on, on account of their similar leg structures and (of course) their
hooves. Even the names of the orders - Artiodactyla and Perissodactyla -
were created in tandem, implying that the number of toes was the main difference
between the even- and odd-toed ungulates.

Since the hoof was the defining character of the ungulates, feet were the
focus of researchers trying to decipher their origins. The large flat nails
of elephants, hyraxes, and sea cows - collectively called the "paenungulates"
("almost ungulates") - were thought to represent an evolutionary intermediate
between traditional claw-like nails and true hooves. Originally placed at
the base of the ungulate lineage, continued research suggested that the
paenungulates were more specialized than the true ungulates, nesting them
firmly in the ungulate family tree . . . thereby requiring that the definition
of ungulate be expanded.

Anatomical studies and fossil findings connected these five orders (along
with several extinct groups of ungulates) to a single common ancestor - the
early ungulate order Condylarthra from the Paleocene Epoch (65 to 54.8 million
years ago). It also became apparent that Condylarths had evolved into other
mammals as well, namely the aardvark (Tubulidentata) and the nail-less whales
and dolphins (Cetacea). Because of the (presumed) common ancestry of these
orders, a taxonomic infraorder - Ungulata - was created, encompassing
all descendants of Condylarthra . . . and as a result, the definition of
"ungulate" was expanded to include animals without a single hoof!

UNGULATE: The Modern
Definition

For many years, "ungulates" were grouped together based on a relationship
to an extinct ancestor, not on the basis of hooves. The morphological family
tree (above) was the accepted classification of mammals, and was continually
being fine-tuned as new fossil evidence was uncovered. (The position of cetaceans
next to Artiodactyla was only resolved in the late 1990s). However, recent
advances in molecular technology have painted a vastly different picture
of how the different groups of mammals are related, relying on similarities
in genetic codes rather than appearances. While the first few studies were
greeted with skepticism, the consistency of genetic markers has caused a
paradigm shift in modern mammal taxonomy (see the Molecular Tree below).
This has had two major implications for ungulates:

While the order
Cetacea was thought be from a Condylarth lineage, the strength of this tie
was not realized until genetic research found a strong affiliation with
hippopotamuses. In fact, based on genetic markers from several genes, it
has been determined that Cetacea is actually a SUBSET of the order Artiodactyla.
Merging two well-defined orders together challenged many established beliefs,
and only recently has the result been widely accepted. While some authors have proposed renaming the order "Cetartiodactyla" to reflect the new combination, the general consensus is that the order should retain the name Artiodactyla
(which has priority), with cetaceans nested within.

Molecular research has also caused the Ungulata to evaporate from mammalian
phylogenies. As seen in the family tree to the left, the members of the former
infraorder Ungulata are now widely spaced apart. The "true" ungulates -
Artiodactyla and
Perissodactyla - remain closely allied,
although these groups are far more closely related to pangolins (Pholidota),
bats (Chiroptera), carnivores (Carnivora), and insectivores (Eulipotyphla)
than they are to the paenungulates. Based on molecular AND fossil evidence,
the branch of the tree containing the true ungulates arose on the continent
of Laurasia, and hence, they are titled the "Laurasiatheria". As for the
paenungulates, although they are no longer associated with the true ungulates,
they DO have a strong evolutionary relationship with each other. These four
orders, along with the the elephant shrews (Macroscelidea) and tenrecs and
golden moles (Afrosoricida), form the "Afrotheria", so named due to an
evolutionary history in Africa.

But what of the Ultimate Ungulate
Page?

When this website first started, Ungulata (without cetaceans) was still accepted
as a taxonomically-based (if paraphyletic) grouping. Even as the close ties
between cetaceans and ungulates were established, their specialized nature
meant that there were never any plans to add these "distant relatives" to
the Ultimate Ungulate roster. Cetaceans still represent a highly specialized
group of mammals, and, even though they are now included within an
ungulate order, their presence here would make very little practical sense.
Similarly, there is little justification for including the (unrelated and
un-hoofed) paenungulates on an ungulate website (perhaps the Paenultimate
Paenungulate Page will come into being in the future!?). Which leaves us
with the true ungulates: "Artiodactyls" and Perissodactyls; two groups of
large, herbivorous animals equipped with hooves. Although they no longer
form a good taxonomic group, their similar biologies makes it convenient
to continue using the name "ungulate" in a descriptive sense.
UltimateUngulate.com is thus focussing in on TRUE ungulates with HOOVES,
returning to the initial definition of ungulate, first put in place
when humans started classifying animal life!

Nikaido, M., A. P. Rooney, and N. Okada. 1999. Phylogenetic relationships
among cetartiodactyls based on insertions of short and long interspersed
elements: Hippopotamuses are the closest extant relatives of whales. Proceedings
of the National Academy of Science USA; 96: 10261-10266.